Harpacticoid copepod communities of floating seaweed: controlling factors and implications for dispersal

The structure of harpacticoid copepod communities of floating detached intertidal algae was studied by surveys and experiments. Seaweed clumps were sampled from Faxaflói, western Iceland, at various distances from the shore (0.2–14 km). Altogether 20 species of harpacticoids were found in these clumps, most of them typical phytal species. Multidimensional scaling ordination (MDS) separated the samples into three distinct clusters in which the communities were significantly different from each other. The distance from shore did not seem to affect the community structure. However, diversity and abundance of harpacticoids increased with size and complexity of the clumps. Experimental Ascophyllum nodosum fronds placed 300–600 m offshore were rapidly colonised by harpacticoids. Density and species diversity were, after 20 days afloat, higher than in the free-floating clumps. Species of the genus Harpacticus were successful colonisers, in particular Harpacticus chelifer and Harpacticus uniremis. In a survey of macrofauna (0.5-mm sieve) on floating algae clumps from various locations around Iceland, H. chelifer was present in all clumps studied, up to 117 km offshore. Its sex ratio was skewed towards females, often with a high ratio of ovigerous females. There was, however, no evidence that this species is using the algal clumps as a platform for reproduction.     

A general theory on fish aggregation to floating objects: An alternative to the meeting point hypothesis

An immense variety of fish may, on occasions, aggregate around or beassociated with floating structures such as drifting algae, jelliedzooplankton, whales, floats or anchored fish aggregating devices (ineffect, there are over 333 fish species belonging to 96 familiesrecorded in the literature).Several hypotheses have been advanced to explain this behaviour ofpelagic fish, although the most widely accepted theory is that fish usefloating materials, to some extent, to protect themselves frompredators. However, we think that aggregation under floats may be theresult of behaviour that has evolved to safeguard the survival of eggs,larvae and juvenile stages, during dispersion to other areas. Naturalfloating structures (e.g., algae, branches of trees) drift in seacurrents that originate in places where the floating objects arefrequently found (e.g., river estuaries, coastal areas). These same seacurrents also introduce some of the planktonic production generated inthese areas into the oligotrophic pelagic environment. Fish associatedwith drifting floating structures probably feed on invertebratesassociated with the structures. However, they may also benefit from theaccumulated plankton in the converging waters. Adult fish of somemigratory species (tuna, dolphinfish, etc.) have also developed similarassociative behaviour around drifting objects for other reasons (e.g.,resting places, presence of bait fish, geographical references andschool recomposition). In this context, the meeting point hypothesis isonly applicable to one specific case, the tuna and tuna-like species.Aggregative and associative behaviour, under and around floatingdevices, may be the result of convergent behaviors that result fromdifferent motivations. However, generally this behaviour can beexplained by the fact that drifting floating objects represent a meansof reaching relatively rich areas, where larvae and juvenile fish havean increased chance of survival.     

Improvement of Aconitum napellus micropropagation by liquid culture on floating membrane rafts

An efficient method was developed using floating membrane rafts (Liferaft) for the micropropagation of Aconitum napellus (Ranunculaceae), a cut flower crop with a low natural propagation rate. This was achieved by introducing shoot tips into culture on Murashige and Skoog's (1962) solid medium, or liquid medium-supported rafts, supplemented by different levels of benzyl adenine (BA). Optimum shoot proliferation on solid medium required 4mg/l BA, whereas for expiants supported on rafts optimal proliferation was achieved at 0.25mg/l BA. Maximum shoot proliferation was found using the floating rafts (propagation ratio of 4.2 per month), 45% higher than the maximum value on solid medium. A similar value could be obtained on solid medium after a period of 2 months. The optimal response to BA was similar for fresh weight gain and shoot length. Growth in a shallow layer of liquid in shake flasks gives a similar shoot multiplication rate to that on floating rafts; however, submerged leaves brown and die.Abbreviations BA 6-benzylaminopurine - GA₃ Gibberellic acid - IBA indole-3-butyric acid - IAA indole-3-acetic acid - NAA naphthalene acetic acid     

The relative importance of water quality, sediment composition and floating vegetation in explaining the macrobenthic community structure of floodplain lakes (Paraná River, Argentina)

The bottom macroinvertebrates of three floodplain lakes were sampled during a period of 15 months to evaluate the relative importance of water quality and sediment-vegetation variables in explaining patterns of variation in community structure. Water quality variables were associated with hydrological and climatic cycles, while sediment-vegetation variables were related to local environmental conditions. The relative weights of both sets of variables were measured by performing several Partial Canonical Correspondence Analyses (PCCA), followed by a variance partitioning technique. In all lakes, the greatest macrobenthic biomass was observed during the low water phase. During that period, Chironomus and burrowing mayflies (Campsurus) were dominants in a lake having 60% cover of Eichhornia crassipes whereas sphaeriid clams (Pisidium and Eupera), burrowing mayflies, and gastropods (Littoridina) were more abundant in a lake without floating plants. A lake completely covered with E. crassipes had the lowest mean biomass. Biomass decreased sharply during the high water period, and community structure became similar in all lakes, with dominance of Oligochaeta and Ostracoda. The most relevant environmental variables associated with spatial and temporal variations were dissolved oxygen, calcium, clay, floating vegetation, and bottom organic matter. Environmental variables explained about 74% of total variance in community structure, with similar relative contributions of physical and chemical variables of the water (16%), and substrate-vegetation variables (14%). Both sets of variables shared 44% of the explained variance. These results suggest that in the studied lakes, the flood is the major batch process affecting macrobenthos, mainly through changes in water quality. However, local environmental conditions may be as important as water quality in explaining among-lakes variations in macrobenthos at low water periods.     

迷走神经对家兔在体心脏心室肌细胞跨膜电位的影响

本研究观察了电刺激迷走神经对家兔在体心脏心室肌细胞跨膜电位的作用及钾通道阻滞剂氯化四乙基铵对这一作用的影响。结果表明,在自然心率条件下,迷走神经刺激可使静息电位(RP)、动作电位振幅(APA)和0相最大上升速率(dv/dt)_(max)增加,动作电位时程(APD)缩短。冠脉注射氯化四乙基铵使心室肌细胞复极过程明显延长,迷走神经刺激不再引起 RP、APA 增大,动作电位时程不再缩短,(dv/dt)_(max)反而减小。这些结果提示,迷走神经刺激对正常心室肌细胞跨膜电位的影响可能是通过外向 K~ 流增加引起的。     

Primary cultures of hepatocytes on human fibroblasts

Summary Parenchymal hepatocytes isolated from adult rats were cultured on three types of collagen-containing substrata: collagen-coated plates, collagen membranes and confluent diploid human fibroblasts. Hepatocytes on the latter two substrata maintained characteristic morphology for at least 10 days in culture, whereas degenerative changes (cell death and formation of multinucleated hepatocytes) and growth of nonparenchymal elements were seen after 5 days in cultures on collagen-coated plates. Parallel findings were seen on basal and induced levels of cytochrome P-450 and NADPH-cytochrome C reductase. The basal levels of cytochrome P-450 were not measurable after day 3 in hepatocytes cultured on collagen-coated plates, whereas measurable levels were maintained in the hepatocytes cultured on the other two substrata. Addition of phenobarbital or methylcholanthrene at day 5 in culture caused an increase in cytochromes P-450 and P-448, respectively, only in hepatocytes cultured on collagen membranes and confluent fibroblasts. Analogous results were seen for the enzyme NADPH-cytochrome C reductase. The similarities in performance between hepatocytes on collagen membranes and on human fibroblasts show that a continuous collagen-containing substratum is important for optimal performance of hepatocytes in primary culture. The possible importance of cultures of hepatocytes on human fibroblasts for carcinogenesis studies is discussed.     

Ecology of a tidal marsh corixid,Trichocorixa verticalis (Insecta,Hemiptera)

The high marsh panne ecosystem at Adam's Point, N. H. provides a particular opportunity for holistic, yet autecological approach to the natural history of Trichocorixa verticalis var. sellaris (Corixidae). This area was dominated by physical and chemical factors associated with highly variable salinities (1 to 160) and temperatures (–1 to 40°C) caused by irregular tidal flooding and neap tidal dry periods. Trichocorixa verticalis reach a maximum density of 26,914/m² (x = 3,909.6/m²) in open shallow pannes, where eggs hatch in staggered sequence, allowing population replenishment after drought and reflooding. Winter eggs hatched between salinities of 0 to 30 at 20°C and 0 to 20 at 10°C. Summer eggs hatched between salinities of 0 to 55 at variable temperatures (20 to 36°C). All instars showed a similar higher summer and lower fall tolerance; immatures were more tolerant than adults. Body molt measurements of the 6 instars are given here.Aside from being herbivores and detritivores, T, verticalis feed on chironomid larvae and oligochaetes. They are food for invertebrates and vertebrates, and provide substantial food for migratory shorebirds.The experimental work was based on a thesis submitted to the Graduate School of the University of New Hampshire in partial fulfillment of the requirements for the degree of Doctor of Philosophy.     

Biological control of three floating water weeds,<Emphasis Type="Italic">Eichhornia crassipes,Pistia stratiotes</Emphasis>, and <Emphasis Type="Italic">Salviniamolesta</Emphasis> in the Republic of Congo

Since 1999, four specific weevils (Coleoptera, Curculionidae) were released in the Republic of Congo against three exotic floating water weeds: Neochetina eichhorniae Warner and N. bruchi Hustache against water hyacinth, Neohydronomus affinis Hustache against water lettuce, and Cyrtobagous salviniae Calder and Sands against water fern. Recoveries of exotic weevils were made from all 24 release sites except one, and all four species have established and spread (up to 800 km for water hyacinth weevils). Within a few years of releases, control of water fern and water lettuce was such that fishing and navigation could be resumed, while reductions of water hyacinth populations were only beginning.     

A Rapid and Efficient Method for In Vitro Screening of Taro for Leaf Blight Disease Caused by Phytophthora colocasiae

Taro leaf blight caused by Phytophthora colocasiae presents the single biggest constraint for taro cultivation globally. To accelerate breeding and selection for disease resistance to leaf blight, it is important to develop bioassays which could differentiate resistant and susceptible cultivars efficiently. In this study, thirty taro accessions and four released cultivars were evaluated for resistance to leaf blight using a modified floating leaf disc assay. A novel method for mass production of P. colocasiae zoospores was developed and used as inoculum for the assay. There were significant (P < 0.05) differences among accessions in their response to P. colocasiae infection in the detached leaf assay. The accessions could be efficiently classified into various resistance groups based on a 0–4 score. Also, the assay results were consistent with the field evaluation scores of taro accessions. Thus, this study reports the development of a rapid, simple and repeatable assay that can be used to screen large numbers of taro cultivars for resistance to P. colocasiae.     

Epithelial cell interaction in air-liquid interface culture

Summary A novel culture method has been developed to study the interaction of epithelial cells in the absence of a solid substratum. Starting with either a single cell suspension or aggregates, cells were floated at the interface of air and liquid culture medium. Two epithelial cell lines have been studied in this system: Madin-Darby canine kidney cells (MDCK), and a rat bladder tumor cell line (NBT-II). Starting with a single cell suspension of MDCK, the floating cells coalesced in 24 h into sheets of cells. The cells were morphologically polarized with the apical surface facing the liquid medium. Domes were observed regularly in these sheets of cells. NBT-II cells migrated actively from aggregates at the air-liquid interface. In this floating culture, NBT-II cells produced extensive cell processes similar to those seen in cells grown on a solid surface. Because cells at the air-liquid interface lack a solid substratum for adhesion, cell membrane processes such as lamellapodia, retraction fibers, pseudopods, and long, intercellular connections can only exert a tension equal to or less than the surface tension of the liquid. Dimethyl sulfoxide 2% stimulated desmosome formation in floating NBT-II cells, resulting in a cribriform pattern in the sheet of cells. This method of interface can lead to new understanding of morphogenesis of epithelial cells, and the mechanism, of cell motility and formation of cell processess. This research was supported by research grant CA14137 from the National Institutes of Health, Bethesda, MD, and in part by the W. W. Smith Charitable Trust, Rosemont, PA